As the inside of grinding mill drums is subject to substantial impact during operation, all or almost all large-scale grinding mills include protective and replaceable liners that cover the inside of the drum. Usually, the liners are cast from metal and bolted to the mill drum by at least two bolts that traverse the liner, wherein the bolts are typically kept in place by a nut applied from the outside of the drum. For example, exemplary liner segments that are directly and indirectly attached to drum shell are described in U.S. Pat. No. 4,270,705. To reduce deformation, liner segments with small circumferential length can be employed as shown in EP 1 952 887 A1, which increases the number of bolts required. Interlocking protective tiles and matching fastener elements are depicted in U.S. Pat. Nos. 6,189,280 and 6,343,756.
There are numerous bolts suitable for coupling the liners to the drum shell, which may include ordinary bolts or those with one or more specialized structures. For example, U.S. Pat. No. 4,018,393 shows a bolt with enlarged surface contact area, and U.S. Pat. App. No. 2008/0197640 depicts improved bolts that can be removed at an angle.
To detach a worn liner, the nuts are removed using an impact wrench, the bolts are pushed inside the drum, and the liner plates are knocked out of the shell through so called knock-out holes. In most cases, nut and bolt removal is achieved using a hydraulically or pneumatically actuated bolt removal tool (BRT). Alternatively, where operation of the BRT is not practical or possible, the bolt can be removed using a sledgehammer. However, considering the size of many mills (e.g., ball mills up to 26 ft. and SAG mills up to and above 40 ft. diameter), bolts (e.g., 2 in. diameter (M48)) and liner weight 2-6 tons, the use of a sledgehammer as a removal tool is not only tedious and hazardous, but also time consuming. Due to the process critical nature of the milling in mining and other operations, downtime must be minimized to maintain profitability. There are numerous BRTs known in the art, and exemplary BRTS are described in U.S. Pat. No. 6,904,980, WO 2007/000019, and U.S. Pat. App. No. 2009/0126177. However, regardless of the manner of bolt removal using such tools, removal of bolts remains challenging, especially where an operator can not readily access the bolts with a BRT. For example, operational difficulties are compounded where the grinding mill drum has a gearless motor drive. In many cases, the gearless motor drive is located on a non-edge position of the mill shell and so covers a substantial part of the shell. Unfortunately, currently known and commercially available liner segments are configured such that the liner bolts are located under the cover of the gearless motor drive and are generally not accessible to the BRT. Consequently, most mill operators resort to use of a sledgehammer in a confined space. As is readily apparent, such operation is once again tedious and time consuming.
Alternatively, to reduce downtime caused by bolt removal, a robotic system can be used as described in US 2007/0180678. Here, the system operates with a robotic arm and tool that automates the above bolt removal process. While such system generally allows for more rapid bolt removal, additional time for installation, programming, and maintenance is required. Moreover, malfunction of such system tends to add substantial delays to the liner removal. To entirely avoid issues associated with bolt removal, boltless liners can be used as described in CA 2305481 where a plurality of plate segments are held together by wedging plates. Here, the impact forces of the balls in the mill together with the particular plate arrangement are thought to stabilize the liner arrangement and to allow use of harder materials than normally used, which extends the projected life time. However, while such liner configurations provide significant advantages with respect to life time and installation, several new disadvantages arise. For example, removal of the plates for replacement is often more complicated as the plates have locked with each other. Moreover, as the wedging process is continuous, the entire liner must typically be replaced even when only a small section of the liner is defective.
Therefore, there is still a need to provide improved mounting arrangements for liners in grinding mills, and especially for grinding mills with a gearless motor drive.